Stroke is the number three cause of death and a leading cause of serious, long-term disability. Treatment of stroke is restricted to thrombolysis within a three-hour window after ictus. There are also no treatments for stroke specifically designed to promote functional recovery. Preliminary data demonstrate that intravenously (iv) injected bone marrow stromal cells (MSCs) one day or one week after onset of symptoms reduces functional deficits associated with cerebral ischemia. This therapeutic benefit lasts for months. Intravenously injected MSCs enter ischemic brain and therein promote increased growth factor expression, e.g., BDNF, bFGF, VEGF. tn tight of these findings, two primary specific aims are proposed. First, MSC therapy is developed preclinically. Aim 1: To select effective doses and to test the safety and toxicity of iv administration of MSCs in young adult and old rodents subjected to stroke. The major hypotheses being tested in this aim are: MSCs injected iv selectively enter lesioned ischemic brain and improve neurological functional recovery; treatment of stroke within a range of MSC doses is safe and effective; therapeutic benefits are dependent on cellular dose, therapeutic window, and animal age. In Aim 2, the molecular and cellular mechanisms responsible for the therapeutic effect of MSC treatment of stroke are investigated. Aim 2: To provide insight into the mechanisms relating MSC treatment to functional recovery, to measure: levels of growth and trophic factor expression (BDNF, bFGF, VEGF) in the ischemic brain, the spatial and temporal profiles and the phenotypic fate of MSCs in ischemic brain, and the response of endogenous proliferating cells in brain to MSC administration. The corresponding major hypotheses tested are: that growth and trophic factor expressions are enhanced in compromised brain by MSC administration; MSCs administered iv are mostly distributed in the boundary zone of the ischemic lesion; with time, the neural phenotype of MSCs increase; the growth and trophic factors produced in the ischemic brain with resident MSCs alter the compromised brain, e.g. MSCs enhance proliferation of endogenous cells in the ischemic brain. A series of experiments are therefore designed to measure: MSC phenotypic changes, cell fusion and the long-term status of resident MSCs, and the proliferation of cells within select regions of the ischemic brain. The goals of this application are to develop MSC therapy as an effective and safe treatment of stroke. The long-term benefits of MSCs reside in an ability to harvest and amplify a patient's MSCs and to re-inject them iv to treat stroke and possibly other forms of central nervous system disorders.